Two areas of any vehicle that I do not believe in trying to "save money" are fuel and brakes.
There are probably some professionals that can properly coat a gas tank with sealant; although with the amount of flaked off sealant that I see in carburetors (stuck in the idle circuit, normally), there are many that have less than perfect results.
Personally, I would first try a couple of large fuel filters in parallel before doing anything to the tank. Change these a couple of times, and if that doesn't clear up the rust, I will change the tank. If a replacement tank is not commercially available, I will pay one of the specialty companies to fabricate one for me. If the cost is "too high" I will sell the vehicle and let someone else deal with it, but no cutting corners around fuel, or other safety issues.
Others opinions may differ.
Jon.
